The present invention relates to a method of reducing carbonate mineral impurities in aqueous phosphate rock slurries. In a particular aspect, the present invention relates to the use of a conditioning agent which selectively inhibits the flotation of phosphate rock with respect to carbonate mineral impurities. The selective inhibition of the phosphate rock allows the carbonate mineral impurities to be concentrated in the froth.
The "Crago" or "double float" froth flotation process, as described by A. Crago in U.S. Pat. No. 2,293,640, Aug. 18, 1942, is commercially used for beneficiating fractions of phosphate ores in which siliceous minerals are the predominant gangue. That process consists of conditioning the material with fatty acid reagents, flotation of the phosphate mineral, deoiling of the concentrate with sulfuric acid to remove the reagents, and refloating with amine reagents to remove the siliceous gangue which either floated or is trapped in the rougher fatty acid flotation.
Some phosphate ores contain carbonate gangue materials in addition to siliceous minerals. The alkaline earth metal carbonate minerals are common impurities in certain ore deposits. Examples of these deposits are the South Florida deposits and the Western Phosphates found in Idaho, Montana, Utah and Wyoming. Such mineral impurities include calcite (CaCO.sub.3), dolomite (CaCO.sub.3. MgCO.sub.3), seashells, aragonite, dolomitic limestone and other less common minerals. The "double float" process has generally been ineffective for removing carbonate mineral impurities from phosphate ore because the flotation characteristics of the carbonate minerals are very similar to those of the mineral phosphates.
Phosphate ores containing undesirable amounts of carbonate mineral impurities greater than 1% by weight must be treated to reduce carbonate mineral impurities to levels below 1% by weight. Carbonate mineral impurities &gt;1% cause problems when the phosphate rock is used for making wet process phosphoric acid. These problems include high acid consumption during the process for preparing wet process phosphoric acid and, an increase in the viscosity of the reaction mixture and the precipitation of sludge forming compounds. The latter two problems are more severe when the carbonate mineral is in the form of magnesium carbonates such as dolomite.
Known methods of reducing the carbonate mineral impurities involve flotation processes wherein a phosphate depressant is added to an aqueous slurry of phosphate rock prior to flotation. Known phosphate depressants include HF, sodium tripolyphosphate, sodium hexametaphosphate, sodium pyrophosphate, fluosilicic acid and orthophosphoric acid.
Problems associated with the above phosphate depressants include high costs and contamination of the water supply preventing reuse of the water in other flotation processes. The present invention remedies the above problems by providing a cheap and contamination-free phosphate rock depressant.